1. Field of the Invention
The present invention relates to electronically controlled vehicle braking systems and more especially to an electronic control for a vehicle parking brake. The park brake system employs an electronic microprocessor control for advanced features along with a backup mode that allows for direct switch control when the electronic control unit has failed that reduces the probability of false apply due to control unit shutdown.
2. Description of the Related Art
Early parking brake systems employed a pedal or lever that was cable coupled to directly actuate certain ones of the vehicle""s hydraulically actuated brake mechanisms, or to apply a separate (dedicated) brake mechanism. A simple approach to electrical control of parking brake systems having a spring apply and a hydraulic or air release is to use a two-position switch to supply current to activate a solenoid actuated valve. In such parking brake systems, the solenoid valve is usually connected in series with a parking brake control that is activated by pushing or pulling a plunger control normally located on a dash of the vehicle. The parking brake is applied by a spring that acts through a linkage to provide a mechanical force that moves friction linings into engagement with a drum or disc brake to effect a parking brake application. In order to release the parking brake, the solenoid valve is actuated to allow pressurized fluid to be communicated to an actuator which acts on a diaphragm or piston associated with the linkage. A hydraulic force developed by the pressurized fluid acts to compress the spring and thereafter move the friction linings away from the drum or disc brake. When energized, the solenoid valve holds pressure in the hydraulic cylinder that in turn holds the spring-actuated brake in the released position. This approach has a minimum number of components but does not allow for more advanced parking brake functions and decisions that can be achieved with bus-connected Electronic Control Units. (ECU""s). The introduction of the ECU in the decision path advances function but can increase the probability of failure or shutdown of the system. Reliability estimates for the standard ECU are significantly lower than those for the direct switch control system. In such an electronically controlled parking brake system, turning off the solenoid valve results in application of the parking brake. Unfortunately, a failure in the computational section of the ECU results in a shutdown of the ECU and all of its actuators and in a shutdown state of the park brake function, the brake applied position. For safety purposes, it is desirable to avoid the inadvertent parking brake application especially while the vehicle is moving.
The present invention provides solutions to the above problems by normally computer controlling an electrical parking brake control signal in response to the condition of an operator actuable apply brake switch and other vehicle operating parameters while monitoring computer operation for continued proper performance. When computer monitoring indicates improper performance, an electrical signal is controlled in response to the position of the apply brake switch independent of computer control and the other vehicle operation parameters.
In the present invention, computer control of the application of electrical energy to a solenoid coil that actuates a parking brake release valve including the testing of computer operation to ascertain erroneous performance. Upon ascertaining erroneous computer performance, computer control of the application of electrical energy to the solenoid coil is relinquished and a manually actuable switch thereafter assumes control of the application of electrical energy to a solenoid coil. The computer control may include selectively applying electrical energy to the solenoid coil in response to the status of a manually actuable switch and the status of other vehicle operating parameters. The steps of relinquishing control and assuming control may be performed by circumventing computer control and coupling the manually operable switch directly in series with the solenoid coil and a source of electrical energy.
An advantage of the present invention is that failure of the electronic control unit does not result in parking brake application at inappropriate times. The probability of the park brake being applied due to failure in the ECU has been reduced by including a backup mode that is entered when the ECU has a failure.